Dichotomous electronic system in a bilayer Ni₁+ nickelate

Summary

Using density functional theory, we calculate the electronic structure of the bilayer infinite-layer nickelate La₃Ni₂O₅F, revealing its ideal two-dimensional character and the coexistence of two distinct quasiparticle behaviors. After treating the oxygen/fluorine disorder with the virtual crystal approximation, band structure calculations show that the conventional Ni dpσ band forms a hole-like Fermi surface, whereas an E* band originating from interstitial density gives rise to a cylindrical electron Fermi surface, resulting in a self-doping of 0.18 electrons. This interstitial density is distributed between the La layers that lack apical oxygen, and as the Ni d_xz/dyz bands approach the M point with parallel linear dispersion, they couple with it to form a nearly non-analytic Dirac point, exhibiting an exotic interstitial-orbital band coupling effect. Concurrently, the d_z² band undergoes a symmetry-driven splitting of approximately 1 eV through interaction with the interstitial density. This dual electron–hole character is expected to govern normal-state transport and far-infrared properties, may influence the superconducting state of nickelates, and offers a fresh perspective for understanding the physics of infinite-layer nickelates.

Materials

• La₃Ni₂O₅F

Methods

• DFT
• full-potential linearized augmented plane wave (FP-LAPW) using WIEN₂k
• virtual crystal approximation

Keywords

• interstitial density
• electron fermi surface
• self doping
• dichotomous quasiparticles
• dirac point
• bilayer nickelate
• electride
• network of valence bands

Highlights

• La₃Ni₂O₅F is an infinite-layer nickelate with formal Ni¹⁺ and no apical oxygen, providing ideal two-dimensional character and a new structural entry into the class of superconducting nickelates.
• The E* band is not derived from atomic orbitals but from interstitial density that extends across three La-apical layers, making it fundamentally different from the conventional dpσ band.
• The material intrinsically hosts two types of quasiparticle Fermi surfaces with opposite signs, constituting a natural two-gap system where the E* electron band may short-circuit the bad-metal resistivity of the dpσ hole band.
• The incipient Dirac point between the interstitial E* and Ni dxz,dyz D* bands represents an unusual type of coupling between delocalized interstitial states and d bands, with sensitivity to chemical doping and strain.

Conclusions

• La₃Ni₂O₅F contains a partially occupied E* band formed by interstitial electron density, creating cylindrical electron Fermi surfaces that result in approximately 0.18 electrons of self-doping, pushing the dpσ hole band away from half-filling and suppressing magnetic ordering.
• The interstitial density couples with Ni dxz,dyz orbitals to form a network of valence bands; near the M point, the E* and D* bands disperse in parallel with identical velocity, constituting an incipient non-analytic Dirac point.
• The system exhibits a dichotomy of hole-like dpσ quasiparticles and electron-like E* quasiparticles, with distinct relaxation times and Fermi surface properties, which will lead to dual behavior in normal state transport and far-IR optical properties.
• Increasing the F fraction or applying compressive strain drives the E* and D* bands to touch at a Dirac degeneracy, hinting at topological character and potential impact on superconductivity.

Main claims

• La₃Ni₂O₅F hosts a partially occupied E* band from interstitial electron density, forming a cylindrical electron Fermi surface that donates ≈0.18 electrons to the dpσ band, resulting in self-doping away from half-filling.
  • Evidence: The E* band… dips from 1.5-2 eV above EF to -0.5 eV at M… forms an electron cylinder around M enclosing 0.18 electrons. As a result, the dpσ bands are not half-filled, with each Ni ion valence becoming +1.09.
• The E* band and the Ni dxz/yz derived D* band approach degeneracy at M, forming an incipient non-analytic Dirac point that can be realized by increasing F fraction (≈0.70) or compressive strain.
  • Evidence: the E* and D* bands at M close up and touch at an F fraction of 0.700-0.705… this 'accidental' degeneracy signals a non-analytic eigensystem with corresponding topological character.
• The two-component electronic system will exhibit opposite-sign quasiparticle contributions in transport, with the E* carriers likely having longer relaxation time and possibly dominating the conductivity.
  • Evidence: The dichotomous electronic structure of La₃Ni₂O₅F involves carriers of opposite sign… dpσ excitations likely will have a short relaxation time… compared to… E* carriers with anticipated longer τE… the E* FS may contribute strongly to the conductivity.
• In the superconducting state, the dpσ Fermi surface will experience magnetic fluctuation pairing while the E* Fermi surface is only indirectly coupled, likely resulting in two isotropic gaps and an extreme two-gap system.
  • Evidence: The superconducting state should show two isotropic superconducting gaps… only the dpσ FS will experience the magnetic fluctuations… the E* FS is at most indirectly coupled to the Ni moment… This picture suggests an ideal, and perhaps extreme, two-gap system.

Workflow

• Structure construction with virtual crystal approximation — The material is an ideal two-dimensional nickelate with Ni₁+ formal valence and isolated bilayer.
  • Materials: La₃Ni₂O₅F
  • Methods: Virtual crystal average for O/F disorder
  • Observations: Ideal two-dimensional infinite-layer structure; No apical oxygen
• DFT electronic structure calculation — Electronic bands reliably obtained, revealing an interstitial band E* not associated with any atom.
  • Methods: First-principles calculations using WIEN₂k (APW+lo)
  • Observations: Band structure computed; Interstitial density identified
• Band structure and Fermi surface analysis — The electronic structure is dichotomous, with a planewave-like interstitial E* band and a correlated dpσ band; the E* band self-dopes the system and couples with d bands to form an incipient Dirac point.
  • Methods: Orbital projection; Fatband analysis; Isosurface plots; Fermi surface mapping
  • Observations: E* band forms electron cylinder around M enclosing 0.18 e; dpσ bands form hole cylinder; Self-doping of Ni valence to +1.09; D* dxz/yz band and E* band approach degeneracy at M; Incipient Dirac point under composition or strain tuning
• Transport and superconducting property predictions — The dichotomous carriers will lead to distinct normal-state transport and far-IR signatures, and an extreme two-gap superconducting state.
  • Methods: Bloch-Boltzmann theory; Relaxation time approximation; Model for Hall and optical conductivity
  • Observations: E* carriers expected to have longer relaxation time; Ma_y short-circuit bad-metal dpσ resistivity; Predicted two-gap superconductivity with isotropic gaps; E* FS only indirectly coupled to pairing